Are you supposed to only be able to bisect once?
As the error said, if you want to use that tool, you need to select some edges/faces that you will bisect. So when you first enter edit mode of the cube, by default all vertices are selected, then after you bisected, nothing is selected.

Anyway, for making simple pyramid, why don't you just make from Cone, with only 4 vertices? Or use the cube, select 4 vertices from top side, then Merge > at Center.